CAMBRIAN AND LOWEE ORDOVICIAN. 119 



Following this synopsis is an account of the conditions of deposition of the sand- 

 stone and of the life of the epoch, from which the following is taken : 



On the southern side of the Archean island the lower P^'i't of the formation usually consists 

 of coarse quartzose sand, of an exceeding open, porous nature, with but little aluminous or ferru- 

 ginous and almost no calcareous matter. Higher in the series the sandstone becomes finer 

 grained and the accessory substances named more abundant. Somewhat above the middle of 

 the series a stratum of shale occurs, attaining a known thickness of 80 feet. This is not every- 

 where present and seems to be mainly developed at some distance from the ancient shore line. 

 It appears to indicate that for a time there was a deepening of the waters, admitting of the accu- 

 mulation of fine sediment, except near the shore, where the deposit of sand continued. Above 

 the shale, sandstone reaching a thickness of 150 feet is again found. This is medium or coarse 

 grained and slightly calcareous. It in turn is overlain by a deposit of associated shale and lime- 

 stone (the Mendota limestone), which attains a thickness of 35 feet in the vicinity of the lake 

 from which it derives its name. These beds indicate a modification of the conditions of deposi- 

 tion, such as to permit not only the settling of fine sediment but the accumulation of calcareous 

 mud as weU. The latter was doubtless derived from the calcareous remains of life, since the sea 

 then swarmed with living organisms whose shells and skeletons are found entombed in the strata. 

 The frequency of broken and worn fragments implies that the greater portion were ground to 

 powder, forming the calcareous flour that subsequently hardened into limestone. These beds 

 appear to point quite surely to a moderate deepening of the waters. 



Overlying this impure limestone is a third and thinner bed of sandstone (the Madison) with 

 which the Potsdam series closes. This, on the whole, is finer grained than that below and is 

 bound more firmly together by cementing material, which is mainly a calcareous and ferruginous 

 infiltration. The thickness of this bed is about 30 feet. 



The "Lower Magnesian limestone" (Prairie du Chien group), which overlies the 

 Cambrian strata, is described by Chamberlin as the equivalent of the " Calciferous" 

 (Beekmantown) of New York, and as a " cherty magnesian limestone, from 65 to 250 

 feet thick, " which " contains but few fossils. " Chamberlin ^^^^ says further : 



During the previous epoch the accumulation of sandstone gave place for a time to the forma- 

 tion of limestone (the Mendota). At the close of the epoch, without any very marked disturb- 

 ance of existing conditions, the formation of limestone was resumed and progressed, with some 

 interruption, till a thickness varying from 65 feet to 250 feet was attained. This variation in 

 thickness is mainly due to irregularities of the upper surface of the formation, which is undulatory 

 and, indeed, in some localities may appropriately be termed billowy, the surface rising and falling 

 like the swells of a subsiding sea. In the localities where these phenomena are best developed, 

 these petrous biUows vary in height from a gentle swell to elliptical domes rising a hundred feet 

 above their bases, while their length ranges from a few rods to a quarter of a mile or more, and 

 their width from one-third to one-half their length. The symmetry of outline here indicated is 

 frequent and typical, though not universal. It finds its best-observed expression in Green Lake 

 and Winnebago counties, where the axes of the domes lie in an east- westerly direction much 

 more commonly than otherwise, or, in other words, are at right angles to the trend of the forma- 

 tion. While not equally conspicuous everywhere, this undulatory surface is prevalent through- 

 out the State and beyond. 



The internal structure of these rock billows is interesting. In the more typical ones at least, 

 and jDcrhaps universally, the superficial strata dip in every direction from the center, most 

 rapidly at the sides (the dip sometimes reaching 30°), and less so at the extremities; or, in other 

 terms, the beds are generally concentric with the surface. The rock of this superficial portion 

 is as homogeneous and even grained a dolomitic limestone as is common to the formation and 

 presents no unusual evidence of fracture or disturbance. Indeed, the rock gives the impression 

 of having been laid down as a mantle of calcareous sediment over an irregular surface. 



